HIV: Treatment, resistance tests

In most developing countries antiviral agents are scarcely available, if at all. Determining resistance in this context is, therefore, not very meaningful. Nevertheless, it is useful to be familiar with the underlying principles.

In the West resistance is determined:

(1) in a pregnant woman before initiating or after failure of therapy when a new treatment is being considered,

(2) upon initiating antiviral therapy in a child,

(3) after failure of therapy when a modification of the therapy is being considered. It should be considered

(4) for an as yet untreated patient before the start of therapy, especially if this is at the time of seroconversion. Viral mutants can in fact “disappear” after the acute phase (they are less easily detected) as they have a lower fitness, in the Darwinian sense of the word. If resistance is not determined in the acute phase of the disease, it is nevertheless advisable to store plasma so that such an investigation can be carried out later.

In (5) post-exposure prophylaxis an attempt should be made to get a sample from the index case.

In each case one should not wait until the results of the resistance determination are known before starting treatment.

However, when the results become available, treatment can be adjusted as necessary.

Techniques

Tests for resistance determination should be carried out with viral RNA derived from plasma, before therapy is stopped and before starting new therapy. It is only possible if there is a detectable viral load (preferably > 1000 RNA copies/ml plasma).

Genotype techniques

• Sequencing: amplification by RT-PCR and automatic sequencing. There is significant interlaboratory variability. In half of the laboratories it has been found that at least 25% of the virus population must consist of mutants before they can be detected.

• LIPA: Line Probe Assay for RT. Detects better some mixed virus populations but can miss certain mutations.

Phenotype techniques

• Virus culture: difficult and time consuming.

• Recombinant virus: the RT and Protease genes of the virus that is to be tested are integrated in a laboratory virus from which these genes have been deleted. This virus is then cultured. With this technique 25% of the virus population must also consist of mutants before any difference in sensitivity can be detected.

• Virtual phenotype: sequencing of the mutant virus and use of a software program to predict sensitivity/resistance.

Category: Medical Subject Notes , Medicine Notes , Microbiology Notes